Physico-mechanical properties together describe the reaction of a particular rock to various types of loads, which is of great importance in the development of wells, construction, mining and other works related to the destruction of rock masses. Thanks to this information, it is possible to calculate the parameters of the drilling mode, choose the right tool and determine the well design.
The physical and mechanical properties of rocks largely depend on the constituent rock-forming minerals, as well as on the nature of the formation process. The reaction of the rock to various mechanical influences is determined by the peculiarity of its structure and chemical composition.
What is rock
A rock is a geological mass formed by mineral aggregates or their fragments, which has a certain texture, structure and physico-mechanical properties.
By texture we understand the nature of the mutual arrangement of mineral particles, and the structure describes all the structural features, which include:
- characteristics of mineral grains (shape, size, surface description);
- features of the connection of mineral particles;
- the composition and structure of cement cement.
The texture and structure together constitute the internal structure of the rock. These parameters are largely determined by the nature of the rock-forming materials and the nature of the geological processes of formation, which can occur both in depth and on the surface.
In a simplified sense, a rock is a constituent material of the earth’s crust, characterized by a certain mineral composition and a discrete set of physical and mechanical properties.
General characteristics of rocks
Rocks can be formed by minerals of different state of aggregation, most often - solid. Rocks from liquid minerals (water, oil, mercury) and gaseous (natural gas) are much less common. Solid aggregates most often have the form of crystals of a certain geometric shape.
Of the 3000 currently known minerals, only a few dozen are rock-forming. Among the latter, six varieties are distinguished:
- clayey;
- carbonate;
- chloride;
- oxide;
- sulfate;
- silicate.
Among the minerals that make up a certain type of rock, 95% are rock-forming and about 5% are accessory (otherwise auxiliary), which are a characteristic impurity.
Rocks can lie in the earth's crust in continuous layers or form separate bodies - stones and boulders. The latter are solid pieces of any composition, with the exception of metals and sand. Unlike stone, a boulder has a smooth surface and a rounded shape, which were formed as a result of running in water.
Classification
The classification of rocks is primarily based on the origin, on the basis of which they are divided into 3 large groups:
- igneous (otherwise called eruptive) - are formed as a result of the rise from the depths of the mantle substance, which hardens and crystallizes as a result of changes in pressure and temperature;
- sedimentary - formed as a result of the accumulation of products of mechanical or biological destruction of other rocks (weathering, crushing, particle transport, chemical decomposition);
- metamorphic - are the result of the transformation (for example, recrystallization) of igneous or sedimentary rocks.
The origin reflects the nature of the geological process, as a result of which the rock was formed, therefore, each type of formation corresponds to a certain set of properties. In turn, the classification within the groups also takes into account the features of the mineral composition, texture and structure.
Igneous rocks
The nature of the structure of igneous rocks is determined by the cooling rate of the mantle substance, which is inversely proportional to depth. The farther from the surface, the magma hardens more slowly, forming a dense mass with large mineral crystals. A typical representative of deep igneous rock is granite.
A quick breakthrough of magma to the surface is possible through cracks and faults of the earth's crust. In this case, the mantle substance quickly hardens, forming a heavy dense mass with small crystals, often indistinguishable to the eye. The most common rock of this type is basalt, which is of volcanic origin.
Igneous rocks are divided into intrusive, which formed in the depths, and effusive (otherwise poured out), which froze on the surface. The former are characterized by a denser structure. The main minerals of igneous rocks are quartz and feldspars.
Sedimentary rocks
4 groups of sedimentary rocks are distinguished by origin and composition:
- clastic (terrigenous) - sediment accumulates from the products of mechanical fragmentation of older rocks;
- chemogenic - are formed as a result of chemical precipitation processes;
- biogenic - are formed from the remains of living organic matter;
- volcanic-sedimentary - formed as a result of volcanic activity (tuffs, clastolavas, etc.).
It is from sedimentary rocks that common minerals of organic origin are extracted that have combustible properties (oil, asphalt, gases, coal and lignite, ozokerite, anthracite, etc.). Such formations are called caustobilites.
Metamorphic rocks
Metamorphic rocks are formed as a result of the transformation of more ancient geological masses of various origins. Such changes are the result of tectonic processes leading to the immersion of rocks to a depth in conditions with higher values of pressure and temperature.
The movements of the earth's crust are also accompanied by the migration of deep solutions and gases, which interact with minerals, causing the formation of new chemical compounds. All these processes lead to a change in the composition, structure, texture and physical and mechanical properties of rocks. An example of such a metamorphism is the transformation of sandstone into quartzite.
General characteristics of physical and mechanical properties and their practical significance
The main physical and mechanical properties of rocks include:
- parameters describing the deformation under the action of various loads (ductility, buoyancy, elasticity);
- reactions to solid intervention (abrasiveness, hardness);
- physical parameters of the rock mass (density, water permeability, porosity, etc.);
- reactions to mechanical stress (fragility, strength).
All these characteristics make it possible to determine the rate of rock destruction, the risk of collapses and the economic cost of drilling.
Data on physico-chemical properties play a huge role in the mining of common minerals. Of particular importance is the nature of the interaction of the rock with the drilling tool, affecting the efficiency and wear of the equipment. This parameter is characterized by abrasiveness.
Unlike other solids, in rocks the physical and mechanical properties are characterized by unevenness, that is, they vary depending on the direction of the load. This feature is called anisotropy and is determined by the corresponding coefficient (Kan).
Density characteristics
This parameter includes 4 parameters:
- density is the mass of a unit volume of only the solid component of the rock;
- bulk mass - calculated as density, but taking into account the existing voids, which include pores and cracks;
- porosity - characterizes the number of voids in the rock structure;
- fracture - shows the number of cracks.
Since the mass of air cavities is negligible compared with solid matter, the density of porous rocks is always greater than the bulk mass. If apart from the pores in the rock there are cracks, this difference increases.
In porous rocks, the value of bulk density always exceeds density. This difference increases in the presence of cracks.
Other physicochemical properties of rocks depend on the amount of voids. Porosity reduces strength, which makes the rock more susceptible to fracture. Nevertheless, such a mass is rougher and more damaging to the drilling tool. Porosity also affects water absorption, permeability and moisture capacity.
The most porous rocks are sedimentary in origin. In metamorphic and igneous rocks, the total volume of cracks and voids is very small (no more than 2%). The exception is several breeds classified as overflowing. They have a porosity of up to 60%. An example of such rocks are trachytes, tuff lavas, etc.
Permeability
Permeability characterizes the interaction of the drilling fluid with rocks during the drilling of wells. This property category includes 4 features:
- filtering
- diffusion;
- heat exchange;
- capillary impregnation.
The first property of this group is decisive, since it affects the degree of absorption of the drilling fluid and the destruction of rocks in the perforation zone. Filtration causes swelling and loss of stability of clay rocks after the initial opening. Based on this parameter, oil and gas production calculations are based.
Strength
Strength characterizes the ability of a rock to withstand destruction under the influence of mechanical stress. Mathematically, this property is expressed in the critical stress value at which the rock is destroyed. This value is called tensile strength. In fact, he sets the threshold of exposure, until which the rock is resistant to a certain type of load.
There are 4 types of tensile strengths: bending, shear, tension and compression, which characterize the resistance to the corresponding mechanical loads. In this case, the impact can be single-ended (one-sided) or multi-bridge (occurs from all sides).
Strength is a complex value, which includes all the limits of resistance. Based on these values, a special passport is constructed in the coordinate system, which is an envelope of stress circles.
The simplest version of the graph takes into account only 2 values, for example, tension and compression, the limits of which are laid off on the axes of abscissa and ordinates. Based on the obtained experimental data, Mohr circles are drawn, and then a tangent to them. The points inside the circles on such a graph correspond to the stress values at which the rock is destroyed. A full strength certificate includes all kinds of limits.
Elasticity
Elasticity characterizes the ability of the rock to restore its original shape after removing the deforming load. This property is characterized by four parameters:
- modulus of longitudinal elasticity (otherwise Young) - is a numerical expression of the proportionality between the stress values and the longitudinal strain caused by it;
- shear modulus - a measure of proportionality between shear stress and relative shear strain;
- bulk elastic modulus - it is calculated as the ratio of stress to relative elastic strain over volume (compression occurs uniformly from all sides);
- Poisson's ratio - a measure of proportionality between the values of relative deformations occurring in different directions (longitudinal and transverse).
Young's modulus characterizes the stiffness of the rock and its ability to resiliently load.
Rheological properties
These properties are otherwise called viscous. They reflect a decrease in strength and stress as a result of a long-term load and are expressed in two main parameters:
- creep - characterizes a gradual increase in deformation at constant stress;
- relaxation - determines the time to reduce stresses arising in the rock during continuous deformation.
The creep phenomenon manifests itself when the value of the mechanical effect on the rock is less than the elastic limit. In this case, the load should be quite long.
Methods for determining the physical and mechanical properties of rocks
The definition of this group of properties is based on the experimental calculation of response to loads. For example, to establish tensile strengths, a rock sample is compressed under pressure or stretched, figuring out the level of impact that leads to failure. The elastic parameters are determined by the corresponding formulas. All these methods are called loading by physical indenters in a laboratory environment.
Some physical and mechanical properties can also be determined under natural conditions using the prism collapse method. Despite the complexity and high cost, this method more realistic determines the response of the natural geological massif to the load.